macOS Server 5.4 running on High Sierra (macOS 10.13) has an adaptive firewall built in, or a firewall that controls incoming access based on clients attempting to abuse the server. The firewall automatically blocks incoming connections that it considers to be dangerous. For example, if a client attempts too many incorrect logins then a firewall rule restricts that user from attempting to communicate with the server for 15 minutes. If you’re troubleshooting and you accidentally tripped up one of these rules then it can be a bit frustrating. Which is why Apple gives us afctl, a tool that interacts with the adaptive firewall.

Once started, the most basic task you can do with the firewall is to disable all of the existing rules. To do so, simply run afctl (all afctl options require sudo) with a -d option:

/Applications/Server.app/Contents/ServerRoot/usr/libexec/afctl -d

You’ll receive no response on successful runs. When run, the adaptive firewall’s rules are disabled. To re-enable them, use the -e option:

/Applications/Server.app/Contents/ServerRoot/usr/libexec/afctl -e

Turning off the rules seems a bit much for most troubleshooting tasks. To remove a specific IP address that has been blacklisted, use the -r option followed by the IP address (rules are enforced by IP):

And to remove a machine, use -x. To understand what is going on under the hood, consider this. The blacklisted computers are stored in plain text in /var/db/af/blacklist and the whitelisted computers are stored in the same path in a file called whitelist. The afctl binary itself is stored in /usr/libexec/afctl and can also be enabled with the /System/LIbrary/LaunchDaemons/com.apple.afctl.plist, meaning to force-stop the service outright, use launchctl:

The configuration file for afctl is at /etc/af.plist. Here you can change the path to the blacklist and whitelist files, change the interval with which it is run, etc. Overall, the adaptive firewall is a nice little tool for macOS Server security, but also something a number of open source tools can do as well. But for something built-in and easy, worth using.
There’s a nice little command called hb_summary located in /Applications/Server.app/Contents/ServerRoot/System/Library/CoreServices/AdaptiveFirewall.bundle/Contents/MacOS that provides statistics for blocked hosts. To see statistics about how much the Adaptive Firewall is being used, just run the command with no options:

The output provides the following information (helpful if plugging this information into a tool like Splunk):

Date

Date statistics start

Number of hosts blocked

Addresses blocked

Number of times each address was blocked

Last time a host was blocked

Total number of times a block was issued

Finally, there are scripts located in /Applications/Server.app/Contents/ServerRoot/usr/libexec that can be used to manage the firewall as well. These include ServerFirewallPromotion.sh (a simple bash script) and ServerFirewallServiceCleanser, a compiled binary.